IMO all debris fields should disappear, and after just one harvest.
Being able to salvage multiple times, with diminishing returns adds very little decision making; it's just extra clicks - complexity for its own sake.
This would also solve the issues caused when debris fields overlap (and thereby remove the needed for them to coalesce).
A perfect case of where streamlining has significantly more upsides, than downsides IMHO.
While I kind of agree (and I've run into that problem with overlapping debris fields myself), I've found some of my best finds from research stations after the 2nd or 3rd Salvage.
One possible compromise would be rather than exiting to the solar system map after collecting the loot from a salvage, it loops back into the salvaging screen directly allowing you to have another go with the reduced chances (as many times as you want), and then once you choose to finish salvaging and exit this screen, the debris field will then permanently dissipate off the system map.
Alternatively if it was streamlined into a single salvage, perhaps give several options with varying consequences and difficulty curves representing progressively more thorough salvaging.
It might be made a little more interesting if Salvaging also suggested at the presence of rare finds (non-commodities), but gave a much increased difficulty for the safe extraction of them; with more risk, not just to yourself, but the inability to safely extract the rare item/s could end up damaging or destroying them.